acmp/cas retries and null arguments

[Aleksey Shipilev]

Hi,

I have a question about read barriers and null handling. It seems to me
that in acmp/cas comparison, if one of the arguments is null, and
comparison had failed, this means it had failed for good, i.e. it is
never a false negative. I.e. comparing the non-null from-copy with null
produces the same result as comparing the non-null to-copy with null.
Comparing null with null would not trigger a false negative.

If so, I wonder if we can coalesce the null checking code in acmp/cas
retries with the actual retries, instead of jumping over the barrier. We
currently do, e.g. in acmp (in some weird linear pseudo-assembly):

acmp(%a, %b):
  test %a, %b
  je <true>

    test %a, %a
    je <anull>
    mov %a, (%a - 8);
    anull:

    test %b, %b
    je <bnull>
    mov %b, (%b - 8);
    bnull:

    test %a, %b;
    je <true>

    ret false;

  true:
  ret true;

...while instead we could generate:

acmp(%a, %b):
  test %a, %b
  je <true>

    test %a, %a
    je <false>  // %a is null, guaranteed "false"
    mov %a, (%a - 8);

    test %b, %b
    je <false>  // %b is null, guaranteed "false"
    mov %b, (%b - 8);

    test %a, %b;
    je <true>

    false:
    ret false;

  true:
  ret true;

This saves read barriers if one of the arguments is not null.

Thanks,
-Aleksey